An EGR may be included with an engine to help reduce engine emissions and increase engine efficiency. In some systems EGR may be cooled via a cooler that is in communication with the engine exhaust passage and the engine intake manifold. The EGR system may further include a bypass valve for directing EGR around the EGR cooler such that EGR is directed from the exhaust passage to the engine intake manifold. Thus, the EGR system can provide cooled or exhaust gas temperature EGR gas to the engine depending on engine operating conditions to improve engine emissions and fuel economy. However, it may be possible for the EGR cooler and/or EGR cooler bypass valve to degrade during some conditions. For example, it may be possible for the EGR bypass valve to remain in an open or closed position when it is desired for the EGR bypass valve to assume the opposite position. Further, since EGR may contain soot, it may be possible for soot to accumulate in the EGR cooler causing the cooling capacity of the EGR cooler to degrade.
Some EGR cooling systems use an EGR model in an attempt to determine whether or not an EGR system having an EGR cooler and an EGR cooler bypass valve is operating as desired. The EGR system model may attempt to assess the operating efficiency of the EGR cooler and EGR valve position based on EGR cooler inlet and outlet temperatures. However, EGR system models can require extensive calibration time and may not agree well with the physical system during some operating conditions. For example, immediately after opening an EGR bypass valve to allow cooled EGR to flow to the engine intake system, the EGR temperature estimate may not agree with the measured EGR temperature since it may be difficult to determine how much heat has been extracted from the exhaust gases in the EGR cooler while untreated exhaust gases were flowing to the engine intake manifold. As such, a difference between the model based EGR temperature and the actual EGR temperature may result in an indication of EGR cooling system degradation.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for diagnosing an EGR system. One example of the present description includes an EGR system diagnostic method, comprising: operating an engine with an EGR bypass valve in a first state for a time greater than a threshold amount of time; indicating a condition of EGR cooler system degradation in response to a request to transition the EGR bypass valve to a second state and a temperature difference between an actual EGR gas temperature and an expected EGR gas temperature before transitioning the state of the EGR bypass valve and at a time greater than the threshold.
By operating an EGR system having an EGR cooler and an EGR bypass valve for a threshold amount of time before comparing an actual EGR gas temperature to an expected EGR gas temperature, it may be possible to determine whether or not an EGR system is operating as desired with little calibration effort. For example, an actual EGR gas temperature may be compared to an expected EGR gas temperature after a threshold amount of time has elapsed. The threshold amount of time may correspond to an amount of time for EGR gases to equilibrate to a temperature after a change in the EGR bypass valve position. Thus, rather than modeling and calibrating an EGR cooler and EGR bypass valve, an empirically determined table or function of EGR gas temperature values may be used as a basis for determining EGR system degradation.
The present description may provide several advantages. In particular, the approach can reduce the amount of time for calibrating EGR system diagnostics. In addition, a simplified diagnostic may be provided by the approach described herein. Further, in some examples, the approach may diagnose EGR system degradation based on parameters other than EGR temperature so as to provide additional sources of EGR system operational verification.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.